1. Field of Invention
This invention relates to a signal conditioner used for a control apparatus which controls industrial processes, such as encountered in oil refineries, petrochemical facilities, steel making, paper making, etc.
2. Description of the Prior Art
In such industrial processes, raw materials are processed and fuels are used, and parameters, such as temperature, flow rate, pressure, liquid level, etc, at various locations in the plant, are measured with sensors. Actuators, such as valves, ancillary to the plant are controlled so that the parameters are held within appropriate ranges. This is important where manufactured products need to be of uniform quality. In order to provide such control, a control system consisting of mainly one or more computers has enjoyed wide acceptance.
FIG. 24 depicts locations of signal conditioners in the control system described above. The term "signal conditioner" is another term for "signal converter". FIG. 24 shows a signal conditioner 4 for receiving signals, and another signal conditioner 5 for transmitting signals. The roles of these signal conditioners are as follows. Receiving signal conditioner 4 receives the output signal SA from a transmitter 6 and arithmetically processes it to convert to a form which can be easily treated by control unit 1, consisting mainly of a computer. Transmitting signal conditioner 5 converts a control signal from control unit 1 into a signal SB, which is adapted for long distance transmission, for example, into a current signal of 4 to 20 mA shown in FIG. 6.
In the field of process control, signal SA, obtained by measurement, and signal SB for controlling an acutator 7, are normally used. The currents of these signals normally lie in the range of from 4 to 20 mA, wherein 4 mA represents 0%, and 20 mA represents 100%. For example, when signal SA of 4 mA is produced, due to measurement, it follows that the measured quantity, such as pressure, is 0. When the produced signal SA is 20 mA, then the measured quantity, such as pressure, gives a full scale reading of 100%. If signal SB for controlling the actuator is 4 mA, then the valve is fully closed. If signal SB is 20 mA, then the valve is fully opened. The reason 0% is represented by 4 mA, rather than 0 mA is to distinguish from the case when transmission line is broken and produces a current of 0 mA. Generally, a signal of 4 to 20 mA can be regarded as a low frequency signal which varies slowly with the processed amount or with the amount to which the acutator is controlled.
The conventional signal conditions are now described with reference to FIG. 24. Transmitter 6 is mounted inside a tube located in the process control system. For example, transmitter 6 measures the pressure of a liquid contained in the tube, and converts the value of the pressure into analog electrical signal SA. Receiving signal conditioner 4 arithmetically processes analog signal SA, introduced from transmitter 6, to convert the signal into a form which can be easily treated by control unit 1. The output signal from conditioner 4 is converted into digital form by an analog to digital converter (called "ADC") 2 and supplied to control unit 1.
Two examples of the arithmetic operations performed by receiving signal conditioner 4 on the analog signal, are as follows. Where transmitter 6 measures a pressure, the transmitter converts the measured pressure value into electrical signal SA, for example, of 4 to 20 mA. Only digital coded signals (hereinafter referred to as "digital signals") are intelligible to control unit 1 consisting principally of a computer. The analog signal, of 4 to 20 MA, is not intelligible to control unit 1. Generally, a voltage signal of 0 to 5 V should be applied to ADC 2. If an electric current of 4 to 20 mA is applied to ADC 2, ADC 2 cannot convert the current into digital form. Thus, receiving signal conditioner 4 arithmetically processes its input analog signal, or the current signal SA of 4 to 20 mA, into a voltage signal of 1 to 5 V.
Where transmitter 6 is a temperature transmitter using a thermocouple sensor, it produces a thermoelectromotive force of several millivolts as signal SA. Since this force is a minute voltage, converter 2 is unable to convert accurately the input signal into digital form. Also, the amplitude of the thermoelectromotive force does not have a linear relation to temperature, and signal SA does not of itself directly represent the temperature. Receiving signal conditioner 4 amplifies the thermoelectromotive force introduced from the thermocouple to an appropriate level which then can be operated on by converter 2. Then, signal SA, or the thermoelectromotive force, is converted into a signal which has a linear relation to the temperature. This process is known as linearizing.
Control unit 1 performs arithmetic operations according to the measured value supplied thereto via receiving signal conditioner 4, to appropriately control the process. The obtained digitally coded signal is applied to digital to analog (labelled "DAC") converter 3, where the signal is converted into analog form. The resulting analog signal is fed to transmitting signal conditioner 5, which then arithmetically process the analog signal to convert it into transmitted signal SB of 4 to 20 mA. This signal controls actuator 7, such as a valve, to control the flow rate to an appropriate value.
Receiving signal conditioner 4 is equipped with a power supply, not shown. In particular, transmitter 6 generally needs a supply of electric power, for example, of 24 volts, to be operated. Transmitting signal conditioner 4 is also equipped with electric power supply, not shown.
The conventional signal conditioners have the following disadvantages.
1. Many different kinds of system subcomponents must be used. Specifically, various different kinds of transmitters 6 and actuators 7 are required in order to suitably control the process. For example, needed would be transmitters for measuring temperature, transmitters for measuring flow rates, and transmitters for measuring pressure, which different transmitters are connected to different locations in a plant carrying out the process. The values obtained by the individual measurements are converted into their respective output signals taking different forms, e.g. a voltage of millivolts, a current of 4 to 20 mA. and a current of 10 to 50 mA. Thus, in order to convert the various analog signals into the same kind of signal, for example, a voltage of 1 to 5 volts, after these analog signals are arithmetically processed as mentioned above, a plurality of receiving signal conditioners 4, respectively suited for the individual signal forms of the outputs from the transmitters, are required to be prepared
In addition, one transmitter needs a DC power supply of 24 volts. Another transmitter needs a power supply of 12 volts. A further transmitter requires a constant current of 4 mA, etc. As a result, the number of combinations of various power supplies and various output forms becomes exorbitant. Also, it is necessary that the number of receiving signal conditioners 4 be identical with the combinations. Also, a considerable number of different kinds of transmitting signal conditioners 5 are required to be prepared because the control signals, applied from control unit 1, are matched to the signal forms respectively required by the connected actuators 7. That is, with the prior art, the various receiving signal conditioners 4 and the various transmitting signal conditioners 5 must be prepared separately.
2. Prior art signal conditioners lack flexibility and cannot be easily modified. In the field of process control, specifications are subject to change. However, if input conditions or output conditions for a circuit that processes an analog input signal are modified, the design of the circuit must be changed or some components must be replaced to accommodate the modification. That is, prior art signal conditioners 4 or 5 are not flexible as regards circuit modifications. For this reason, whenever the input conditions or output conditions are varied, dedicated hardware devices must be prepared. Thus, these prior art arrangements cannot rapidly cope with modifications of specifications.
3. Prior art signal conditioners are not adapted to be fabricated into integrated circuits (IC). Also, since a large number of components are involved, miniaturization cannot be realized. In the past, the analog signal, which was obtained by measurement and supplied from transmitter 6 or from control unit 1, was arithmetically processed to convert the input signal into a desired signal. Although analog ICs which perform arithmetic operations on analog signals exist, input resistors, feedback resistors, capacitors, and other external components connected thereto are needed. Thus, it is impossible to substantially reduce the number of components. Accordingly, few advantages are obtained even if they are fabricated in the form of ICs. In this way, the prior art signal conditioners which perform arithmetic operations on analog signals are not adopted to be manufactured in the form of an IC.
4. Prior art signal conditioners cannot appropriately be adapted to be transmitters or actuators having communication function. In recent years, there has been increased demand for transmitters and actuators having communication functions for providing more adequate process control.
A transmitter having communication functions is now described. An ordinary transmitter measures temperature, flow rate, pressure, liquid level, or the like, and converts the measured quantity into a low frequency signal of 4 to 20 mA. A transmitter having communication function carries the information, e.g. the measuring range of the transmitter, sent from the process control system, by modulated wave. This modulated wave is sent to control unit 1 together with the low frequency by use of multiplexing transmission technique. That is, the transmitter has an intelligence function. The modulation can be effected by frequency modulation, frequency shift keying (referred to as "FSK"), or other methods.
To accommodate a transmitter producing a signal that carries multiplexed communication information, receiving signal conditioner 4 must be equipped with a demodulator. Similarly, transmitting signal conditioner 5 must have a modulator to accommodate itself to an actuator capable of receiving signals carrying multiplexed communication information. As described above in heading 1, numerous kinds of system subcomponents must be provided. In addition, where combinations of transmitters and actuators, each of which may or may not be capable of multiplexing communication information, are taken into account, the number of kinds of signal conditioners is large. Hence, the prior art arrangements are not suited for industrial use.